Question

Which among the following salt solution in water shows pH less than 7 ?

• A. $\text{CuCl}_2$
• B. $\text{CH}_3\text{COONH}_4$
• C. $\text{Na}_2\text{CO}_3$
• D. $\text{KNO}_3$

Hint:

Remember the rule of thumb: "The strong parent wins the pH fight." A salt made from a strong acid and a weak base will always create an acidic solution ($\text{pH} < 7$). Here, HCl is the strong acid component that dominates the pH.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify which aqueous salt solution behaves as an acid, meaning its pH will be less than 7 at room temperature.

Step 2: Key Formula or Approach:
The pH of an aqueous salt solution depends on salt hydrolysis.
1. Salts of strong acids and strong bases do not hydrolyze and form neutral solutions ($\text{pH} = 7$).
2. Salts of weak acids and strong bases undergo anionic hydrolysis to form basic solutions ($\text{pH} > 7$).
3. Salts of strong acids and weak bases undergo cationic hydrolysis to release hydronium ions ($\text{H}_3\text{O}^+$), rendering the solution acidic ($\text{pH} < 7$).

Step 3: Detailed Explanation:
Let's evaluate the parental composition and behavior of each salt:
4. $\text{CuCl}_2$ (Copper(II) chloride): Formed from a weak base [$\text{Cu(OH)}_2$] and a strong acid [HCl]. In water, $\text{Cu}^{2+}$ ions undergo cationic hydrolysis:
$$\text{Cu}^{2+} + 2\text{H}_2\text{O} \rightleftharpoons \text{Cu(OH)}_2 + 2\text{H}^+$$ The increase in $\text{H}^+$ ion concentration makes the solution acidic, so $\text{pH} < 7$.
5. $\text{CH}_3\text{COONH}_4$ (Ammonium acetate): Formed from a weak acid [$\text{CH}_3\text{COOH}$] and a weak base [$\text{NH}_4\text{OH}$]. Since both have nearly identical dissociation constants ($K_a \approx K_b$), the solution remains nearly neutral ($\text{pH} \approx 7$).
6. $\text{Na}_2\text{CO}_3$ (Sodium carbonate): Formed from a strong base [NaOH] and a weak acid [$\text{H}_2\text{CO}_3$]. It undergoes anionic hydrolysis to yield $\text{OH}^-$ ions, making the solution basic ($\text{pH} > 7$).
7. $\text{KNO}_3$ (Potassium nitrate): Formed from a strong base [KOH] and a strong acid [$\text{HNO}_3$]. It does not undergo hydrolysis, resulting in a perfectly neutral solution ($\text{pH} = 7$).

Step 4: Final Answer:
The aqueous solution of $\text{CuCl}_2$ has a pH less than 7, which corresponds to option (A).